Fig. 2

a A bilateral acetabular protrusion developing secondary to RA; the bilateral femoral head protruded inward over Nelaton’s line. b Single-stage not session session bilateral total hip arthroplasty (with the placement of a titanium-coated, biopsy acetabular cup; Smith & Nephew, USA) accompanied by acetabular reconstruction using autologous bone. An X-ray taken immediately after surgery showed that the rotational center of the hip joint had returned to the normal anatomical location. The initial stability of the acetabular cup was good. c At the 4-year follow-up, the X-ray revealed complete healing of the bilateral bone graft, without bone resorption, an acetabular protrusion, or loosening